This invention relates in general to electrostatographic imaging, and preferably, to an imaging member having a roughened surface.
In electrostatography, an imaging member containing an insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge in certain areas of the insulating layer while leaving behind an electrostatic latent image in the other areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles (toner) on the surface of the insulating layer. The resulting visible image may then be transferred from the imaging member to a support such as paper. This imaging process may be repeated many times with reusable insulating layers. It is necessary to clean residual toner from the surface of the insulating layer prior to repeating another imaging cycle.
One common method of cleaning is blade cleaning. Elastomer blade cleaning of imaging members is conceptually simple and economical, but raises reliability concerns in mid- and high-volume applications due to apparent random failures. Such random failures justify the reluctance to include blade cleaners in higher volume machines with or without some back-up element.
Alternative cleaning techniques used in higher volume applications include the use of magnetic, insulative and electrostatic brushes. However, such cleaning techniques are also subject to specific or timed failures. These failures include, but are not limited to, photoreceptor filming and permanent impaction of toner particles and toner fragments. Specific failures may, in part, be related to the materials package, e.g., the toner and any additives contained with the toner. These types of blade and cleaning failures can be quite reproducible.
One random failure mode of a cleaning blade may be due to inherent variations or flaws in the material of the blade, which allow stresses and strains with extended copying to locally fatigue the edge of the blade. An additional random failure mode can be local or image related enhancements or reductions in blade/photoreceptor friction which cause unacceptably large tuck-under of a doctor blade edge. A large enough tuck or break in the blade/photoreceptor seal can permit residual toner and other debris to pass under the blade resulting in streaks on the copy. This not only decreases cleaning efficiency, for example by increasing background, but in severe cases can result in catastrophic system failure.
A number of methods have been implemented or proposed to enhance blade/photoreceptor contact properties. One method includes agitation of the blade against the photoreceptor to prevent build-up of material along the contact seal. Another method includes addition of redundant members, such as disturber brushes to loosen or collect debris which might otherwise stress the blade element. These methods increase the mechanical complexity and the cost of the cleaning assembly, and are thus undesirable.
Another method for enhancing blade/photoreceptor contact properties includes the addition of lubricants to the toner, photoreceptor and/or blade. However, this method increases the materials complexity and introduces compatibility problems. This often results in films developing on the photoreceptor which hinder photoreceptor function and degrade image quality.
A further proposal for enhancing blade/photoreceptor contact properties is by roughening of the photoreceptor surface to reduce the blade friction and the blade/photoreceptor contact area. This method may also introduce compatibility problems depending on how the roughened surface is introduced. For example, particulate additives to the bulk of the transport layer to provide roughness through surface asperities can degrade electrical and/or mechanical properties. Surface asperities can be worn away in normal machine copying, limiting any cleaning benefit. Surface roughening can also have direct adverse effects such as the introduction of sites against which toner may become lodged. Photoreceptor surface roughening can also inhibit cleaning by allowing the blade to pass over toner and other surface debris.
One of the most common "predictable" or non-random blade cleaning failures is permanent impaction of toner particles and toner fragments. This type of failure is generally encountered and resolved during program development. It involves material, including toner particles, which becomes impacted onto the imaging surface and adheres with such force that the material cannot be removed by the cleaning elements. Additional debris, including untransferred toner residue and developer and/or toner additives, may become jammed against an asperity on the photoreceptor surface. Repeated passes and extended copy can lead to the build-up of elongated crusty deposits in front of the asperity which eventually print out as spots on the copy.
Various strategies have also been implemented or proposed to deal with this type of blade cleaning problem, including those enumerated above. Additional approaches to the resolution of such problems include the elimination of the material which impacts or builds up in the tail, the inclusion of additives which lubricate and/or scavenge the offending material, and the development of an imaging surface which resists toner impaction and/or buildup.
One source of the problem is flat toner particles which adhere tenaciously to the imaging surface. The flat toner particles are difficult to remove from the surface because they do not provide much of a profile to place force upon to remove. Further, the flat toner particles contact the surface over a larger surface area than "spherical" toner particles, thereby increasing the adhesion force of the flat toner particles to the surface. The problem of removing flat toner particles is of particular concern, since some toner compositions may contain about 25% flat toner particles.
Although certain additives may prevent these problems, they are not always successful. Lubricating additives in toner may result in filming. For example, magnesium and zinc stearate additives have problems of filming. This filming may be due to the additives containing flat particles which adhere strongly to the imaging surface. Materials in paper, such as talc, tend to form impacted talc particles which then lead to talc films. Talc also may cause image blurring leading to deletions because talc can absorb water from air rendering it conductive. Aerosil particles, typically about 0.03 micrometer in average diameter, likewise cause cleaning problems such as filming.
Overcoating layers for electrophotographic imaging members have been proposed for a number of different reasons. U.S. Pat. No. 4,764,448 to Yoshitomi et al. discloses an amorphous silicon photoreceptor having a specific surface roughness attained by polishing the surface using soft abrasive substances. The polished surface prevents image blurring in the photoreceptor. The surface has at least one of the following properties: (i) a mean surface roughness along the center line as measured by a needle type surface roughness tester being 190 Angstroms (0.019.mu.) or less; (ii) a mean surface roughness along the center line as measured by a coordinates measuring scanning electron microscope and a section measuring apparatus being 60 Angstroms (0.006.mu.) or less; (iii) a variance of mean surface roughness along the center line as measured by a coordinates measuring scanning electron microscope and a section measuring apparatus being 70 Angstroms (0.007.mu.) or less; (iv) a maximum surface amplitude as measured by a coordinates measuring scanning electron microscope and a section measuring apparatus being 450 Angstroms (0.045.mu.) or less; and (v) a difference between the mean of five largest values of the surface roughness as measured by a coordinates measuring scanning electron microscope and a section measuring apparatus and the mean of five smallest values of the surface roughness being 420 Angstroms (0.042.mu.) or less.
U.S. Pat. No. 4,904,557 to Kubo discloses an electrophotographic photosensitive member comprising a photosensitive layer having a surface roughness of ten points over a reference length of 2.5 millimeters. The particular surface roughness is provided to prevent an interference fringe pattern appearing at image formation, and for preventing black dots appearing at reversal development.
U.S. Pat. No. 4,537,849 to Arai discloses a photosensitive element having a roughened selenium-arsenic alloy surface. The outer photoconductive surface is roughened by direct mechanical grinding (polishing). A roughness of less than or equal to 3.0 micrometers laterally and from 0.1 to 2.0 micrometers in height is disclosed for reducing adhesion of transfer paper or toner.
U.S. Pat. Nos. 3,992,091 and 4,076,564 to Fisher disclose roughened imaging surfaces of a xerographic imaging member. Roughening of the photoreceptor surface is achieved indirectly by first chemically etching a substrate. The substrate is then uniformly coated with photoconductive material which conforms to the surface in such a way that the substrate roughness is reproduced on the photoconductive surface. The level of roughness may be from 3 to 5 or 10 to 20 micrometers laterally with a 1 to 2 micrometers height.
U.S. Pat. No. 4,134,763 to Fujimura et al. discloses a method for making the surface of a substrate rougher by bringing a grinding stone in light pressure contact with the surface of the substrate. Small vibrations form a minute roughness on the surface of the substrate. The substrate surface roughness is preferably from 0.3.mu.to 2.0.mu.. The rough surface of the substrate improves adhesion between the substrate and a selenium layer. Unlike the Fisher patents, the roughness of the substrate is not disclosed as being reproduced in the imaging surface layer.
U.S. Pat. No. 4,804,607 to Atsumi discloses an overcoat layer which is a film-shaped inorganic material coating the surface of a photosensitive layer. The overcoat layer is formed such that a rough surface is provided having 500-3000 convexities and concavities per 1 cm linear distance with a maximum depth difference of 0.05 to 1.5 micrometers between the convexities and the concavities. The convexities and concavities are formed by heating the support, photosensitive layer and the overcoat layer.
U.S. Pat. No. 4,693,951 to Takasu et al. discloses an image bearing member having a maximum (vertical) surface roughness of 20 micrometers or less, and an average surface roughness which is less than or equal to two times a toner particle size. However, nothing about the wavelength between peaks is mentioned.
While the above described imaging members provide a roughened surface for various purposes, the references do not teach or suggest a particular surface roughness which would be desirable for preventing the adhesion of toner particles, and in particular, toner flat particles and additives in the toner which may result in permanent impaction of toner particles and fragments.